Update the Strapi guide to version 5 and use the official Strapi client library instead of a custom function

[Dovakeidy]

The Astro guide for Strapi is outdated regarding the data format returned by the Strapi API. Since Strapi v5, the response no longer includes the attributes field. Instead, it now uses a documentId field.
👉 See the breaking change documentation for more details.

Additionally, Strapi now provides an official client library that simplifies API calls and should be preferred over writing custom fetch logic.
📚 Official client documentation: Strapi Client

It would be better for the guide to be updated to reflect these changes and leverage the official client.